Radiative relaxation quantum yields for synthetic eumelanin

We report absolute values for the radiative relaxation quantum yield of synthetic eumelanin as a function of excitation energy. These values were determined by correcting for pump beam attenuation and emission reabsorption in both eumelanin samples and fluorescein standards over a large range of concentrations. Our results confirm that eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through nonradiative means. Furthermore, we have found that the radiative quantum yield of synthetic eumelanin is excitation energy dependent. This observation is supported by corrected emission spectra, which also show a clear dependence of both peak position and peak width on excitation energy. Our findings indicate that photoluminescence emission in eumelanins is derived from ensembles of small chemically distinct oligomeric units that can be selectively pumped. This hypothesis lends support to the theory that the basic structural unit of eumelanin is oligomeric rather than heteropolymeric.     

Radiative relaxation in synthetic pheomelanin

We report a detailed photoluminescence study of cysteinyldopa-melanin (CDM), the synthetic analogue of pheomelanin. Emission spectra are shown to be a far more sensitive probe of CDM's spectroscopic behavior than are absorption spectra. Although CDM and dopa-melanin (DM, the synthetic analogue of eumelanin) have very similar absorption spectra, we find that they have very different excitation and emission characteristics; CDM has two distinct photoluminescence peaks that do not shift with excitation wavelength. Additionally, our data suggest that the radiative quantum yield of CDM is excitation energy dependent, an unusual property among biomolecules that is indicative of a chemically disordered system. Finally, we find that the radiative quantum yield for CDM is approximately 0.2%, twice that of DM, although still extremely low. This means that 99.8% of the energy absorbed by CDM is dissipated via nonradiative pathways, consistent with its role as a pigmentary photoprotectant.     

Time-resolved and steady-state fluorescence spectroscopy of eumelanin and indolic polymers

Eumelanin plays a variety of important physiological roles in human skin. However, its structure and fundamental properties still remain poorly understood. Although the absorbance of eumelanin is broad and reveals little about its structure, a variety of techniques have revealed the presence of a disordered array of chromophores within the melanin compound. In order to examine the fluorescence decay dynamics of these chromophores, time-resolved spectroscopy was applied to solutions of synthetic eumelanin and a melanin-like polymer of N-methyl,5-hydroxy,6-methoxyindole (N-Me-5H6MI). Solutions were excited with 80 fs laser pulses at 355, 370, 390 and 400 nm, and decay time courses were acquired at 20 nm intervals between 400 and 600 nm for each excitation wavelength. Decay profiles for both eumelanin and the polymer exhibited a characteristic multiexponential behavior with decay times between 0.5 and 15 ns, although steady-state spectra for the polymer exhibited only two peaks. The long-decay component in the polymer showed a significant decrease in both amplitude (30-5%) and decay time (14-6 ns) with increasing emission wavelength. In contrast, the amplitude and decay time in melanin increased slightly (10-15% and 7-10 ns, respectively) from 400 to 520 nm emission, at which point they leveled off. These trends were consistent for all excitation wavelengths. These results suggest that the multiexponential behavior of melanin fluorescence is characteristic of each oligomer within the eumelanin compound, and is consistent with the assertion that the diversity of constituents within eumelanin provides it with a robustness in spectral properties.     

Fluorescence characteristics of difenzoquat

Difenzoquat is identified as a potentially useful fluorescence probe. Its fluorescence excitation spectrum shows significant concentration-dependent variations not found in the absorption and fluorescence emission spectra. The effects are ascribed to association phenomena that are unaffected by ionic strength and anionic/non-ionic detergents, but strongly promoted by cationic micelles. INDO calculations account for the features of the monomer excitation spectrum, but do not explain concentration dependent behavior.     

An indirect method of X-ray spectra measurement by simultaneous attenuations of the scattered beam

Direct and indirect methods of X-ray spectra determination present obstacles to their practical use since they must position either the collimator-detector assembly or the attenuators-ionization chamber, respectively, along the X-ray beam direction. These arrangements require considerable space and in many instances the detectors promptly saturate. An indirect procedure, which overcomes the aforementioned problems, is developed. It consists of the scattering of the X-ray beam from a carbon disk, which is detected simultaneously by several detectors placed away from the beam. The X-ray flux reaching each of these detectors is attenuated in metal sheets of different thicknesses, thus obtaining simultaneously the attenuation curve values. A set of analytical equations are derived to calculate attenuation curves by taking into account all the absorption and elastic and inelastic scattering processes that a beam of photons undergoes when going from the X-ray tube to the detector. Users, even those who are not well acquainted with computer programming, can easily obtain the X-ray spectrum by a least square fitting of a measured attenuation curve to a previously derived analytical expression. A simulated Monte Carlo program of photon transport from the X-ray tube to the detector provided simulated attenuation curves data. Analytically calculated and simulated attenuation curves for the same input spectrum wholly overlap and furthermore, reconstructed spectra from both sets of curves for different kilovoltages are also in full agreement. Finally, in addition to the importance of having the detectors out of the beam direction, the proposed arrangement features other main advantages, namely, only one X-ray tube shot is needed to obtain the required data, the physical processes involved are very well known, analytical equations are easily interpreted, and the measuring apparatuses can be comparatively simple to assemble and operate.     

A Model for the Interaction of 6-Lauroyl-2-(N,N-dimethylamino)naphthalene with Lipid Environments: Implications for Spectral Properties

Although 6-lauroyl-2-(N,N-dimethylamino)naphthalene (LAURDAN) is now widely used as a probe for lipid systems, most studies focus on the effect of the lipid environment on its emission properties but not on the excitation properties. The present study is intended to investigate the excitation properties of LAURDAN in diverse lipid environments. To this end, the fluorescence properties of LAURDAN were studied in synthetic ester and ether phosphatidylcholines and sphingomyelin vesicles below, at and above the corresponding lipid main phase-transition temperature. The excitation spectra of LAURDAN in these environments always show at least two well-resolved bands. In the different lipid vesicles the behavior of the red band in the LAURDAN excitation spectra is sensitive to the lipid chemical environment near the probe fluorescent moiety and to the packing of the different lipid phases (gel and liquid crystalline). We propose that the interaction between the LAURDAN dimethylamino group and the ester linkage of ester phospholipids is responsible for the strong stabilization of LAURDAN's red excitation band in the gel phase of ester phospholipid vesicles. We discuss the consequence of these proposed ground-state interactions on LAURDAN's emission generalized polarization function. In the context of variable excitation wavelengths, information concerning solvent dipolar relaxation through excitation generalized polarization function is also discussed.     

PHOTOINDUCED OXYGEN CONSUMPTION IN MELANIN SYSTEMS–II. ACTION SPECTRA and QUANTUM YIELDS FOR PHEOMELANINS

Abstract— Photoinduced oxygen consumption in systems containing synthetic and natural pheomelanins has been studied by ESR spectroscopy using a nitroxide spin probe to monitor oxygen concentration. Action spectra and quantum yields have been determined for melanin from 5-S-cysteinyldopa and for pheomelanins extracted from red human hair and red chicken feathers. For comparison, data also were obtained for eumelanins from black hair and black feathers. The action spectrum for oxygen consumption by cysteinyldopa melanin is closely related to that previously obtained for eumelanins except that it shows slightly less efficiency at intermediate wavelengths (ca. 300–400 nm). Action spectra for the natural pheomelanins resemble either that for cysteinyldopa melanin or that for eumelanin. In general pheomelanins are no more effective than eumelanins in promoting oxygen consumption, i.e. they are no more susceptible to net photooxidation.     

Cathodoluminescence efficiency dependence on excitation density in n-type gallium nitride.

Cathodoluminescence (CL) spectra from silicon doped and undoped wurtzite n-type GaN have been measured in a SEM under a wide range of electron beam excitation conditions, which include accelerating voltage, beam current, magnification, beam diameter, and specimen temperature. The CL intensity dependence on excitation density was analyzed using a power-law model (I CL proportional, variant J m ) for each of the observed CL bands in this material. The yellow luminescence band present in both silicon and undoped GaN exhibits a close to cube root (m = 0.33) dependence on electron beam excitation at both 77 K and 300 K. However, the blue (at 300 K) and donor-acceptor pair (at 77 K) emission peaks observed in undoped GaN follow power laws with exponents of m = 1 and m = 0.5, respectively. As expected from its excitonic character, the near band edge emission intensity depends linearly (m = 1) in silicon doped GaN and superlinearly (m = 1.2) in undoped GaN on the electron beam current. Results show that the intensities of the CL bands are highly dependent not only on the defect concentration but also on the electron-hole pair density and injection rate. Furthermore, the size of the focussed electron beam was found to have a considerable effect on the relative intensities of the CL emission peaks. Hence SEM parameters such as the objective lens aperture size, astigmatism, and the condenser lens setting must also be considered when assessing CL data based on intensity measurements from this material.     

Nanocrystals for large Stokes shift-based optosensing

The influence of Stokes shift in optosensing was discussed. Then, the current status of large Stokes shift-based optosensing was reviewed here.     

Nanocrystals for large Stokes shift-based optosensing

Stokes shift is an important feature of fluorescence, which reveals the energy loss between the excitation and the emission. For most fluorescent materials(e.g., organic dyes and proteins), the large overlap between the absorption and emission spectra endow them a small Stokes shift that induced reabsorption by fluorophore itself. Although the self-absorption can be effectively reduced due to the emergence of fluorescent nanomaterials, fluorescence attenuation is still observed in aggregated or concentrated nanocrystals, causing reduced sensitivity of biosensors. Therefore, increasing the Stokes shift can effectively improve the performance of nano-agents based biosensing. In this critical review, through understanding the Stokes shift from the viewpoint of self-absorption, the influence of Stokes shift on fluorescence properties are discussed. Based on the principle of changing the Stokes shift of fluorescent nanomaterials, we described the methods for constructing various optically large Stokes shift-based nanomaterials, and the application of these nanocrystals in biosensing is especially concerned in this review.     

FLUORESCENCE OF MELANIN-DEPENDENCE UPON EXCITATION WAVELENGTH AND CONCENTRATION

Abstract— An introduction to the fundamental characteristics of synthetic melanin fluorescence is presented. The particular difficulties associated with the detection and reduction of the relatively weak signal are discussed and a technique is described for correcting the fluorescence spectra for attenuation of the excitation and emission beams. Spectra are reported for the excitation wavelength range 340–400 nm and an emission range of 360–560 nm. The concentration dependence of the corrected fluorescence signal is examined and is shown to be linear. The variation of the fluorescence spectra with excitation wavelength suggests a two-component fluorescence, for the wavelength range studied. The presence of an isosbestic point in the spectra is used to identify the fluorophores as components of a reaction equilibrium. The possible relationship of this equilibrium to that associated with the melanin photo ESR is discussed     

The use of polarizers to improve detection limits in fluorimetric analysis

The use of variously-oriented film polarizers to reduce scattered light interference in fluorimetric analysis is reported. Single horizontally-oriented polarizers in the excitation beam (to obtain fluorescence spectra) or the emission beam (to obtain excitation spectra) are efficient at removing first-order Rayleigh scattered light signals. Crossed polarizers are necessary to remove substantial second-order signals. With the aid of polarizers, fluorimetric detection limits can be considerably improved, even when excitation and emission maxima are well separated, and when deproteinized blood plasma is analyzed. Polarizers are also effective in reducing scattered light signals from macromolecular solutes.     

Interstitial fluorescence spectroscopy in the human prostate during motexafin lutetium-mediated photodynamic therapy

The in vivo fluorescence emission from human prostates was measured before and after motexafin lutetium (MLu)-mediated photodynamic therapy (PDT). A single side-firing optical fiber was used for both the delivery of 465 nm light-emitting diode excitation light and the collection of emitted fluorescence. It was placed interstitially within the prostate via a closed transparent plastic catheter. Fitting of the collected fluorescence emission spectra using the known fluorescence spectrum of 1 mg/kg MLu in an intralipid phantom yields a quantitative measure of the local MLu concentration. We found that an additional correction factor is needed to account for the reduction of the MLu fluorescence intensity measured in vivo due to strong optical absorption in the prostate. We have adopted an empirical correction formula given by C = (3.1 cm(-1)/micro's) exp (microeff x 0.97 cm), which ranges from approximately 3 to 16, with a mean of 9.3 +/-4.8. Using a computer-controlled step motor to move the probe incrementally along parallel tracks within the prostate we can determine one-dimensional profiles of the MLu concentration. The absolute MLu concentration and the shape of its distribution are confirmed by ex vivo assay and by diffuse absorption measurements, respectively. We find significant heterogeneity in photosensitizer concentration within and among five patients. These variations occur over large enough spatial scales compared with the sampling volume of the fluorescence emission that mapping the distribution in three dimensions is possible.     

Electron spin resonance (ESR/EPR) of free radicals observed in human red hair: a new,simple empirical method of determination of pheomelanin/eumelanin ratio in hair

The definition of the concentration of pheomelanin in the skin is an issue of great interest because in the case of being influenced by UV radiation, it manifests itself as a prooxidant, causing various skin disorders including melanoma that might help to explain the relatively high incidence of skin cancer among individuals with red hair. The ESR spectra of red hair samples were investigated. It was found that at low microwave power, they are characterized by two types of spectra. Red hair ESR signals result from a superposition of two spectral shapes, a singlet spectrum as a result of the existence of eumelanin and a triplet spectrum as a result of the existence of pheomelanin. At high microwave power, only triplet spectra shape was detected, caused by saturation of the eumelanin singlet. Using different concentration ratios of black to red hair, we measured ESR spectra and plotted the ratio values in each sample against a measured ‘g‐factor’ (experimental). We found that there is a linear relationship between these two parameters. So, it is evident that using these results, the concentration ratio of pheomelanin to eumelanin in a sample of hair can be easily determined by an almost noninvasive method. This can be considered a potential advantage for many practical activities compared with other invasive methods. The concentration dependence curve of pheomelanin (µg/mg) on g_exp‐factor in an ESR spectrum of hair has been designed, which allows the determination of the amount of pheomelanin in hair of any color. Copyright © 2014 John Wiley & Sons, Ltd.     

Concentration dependence of the spectroscopic and photochemical properties of atomic and molecular oxygen in argon matrices

Vacuum ultraviolet (VUV) excitation (200-100 nm) and visible emission (300-650 nm) spectra of O2 imbedded in Ar matrices at different concentrations are presented. At 0.1 and 0.2% concentrations a linear increase in the intensities of the excitation and emission spectra is observed. At these concentrations, photolysis of O2 is found to be negligible. At higher concentrations (0.5, 1 and 2%) the normalized intensities of the excitation and emission spectra of O2 decrease. With increasing concentration of O2 the permanent photolysis of O2 increases, which does not correlate, based on the excitation spectra of O in Ar, to the production of isolated O atoms. It has been shown that this anomalous behavior should be due to the formation of van der Waals dimers and oligomers at higher concentrations of O2 that upon photolysis produce ozone.     

YTao4：Nb，Eu的发光性能研究

用高温固相反应法合成了铌酸根NbO^3-4和Eu^3 共掺杂的正钽酸盐化合物Y1－xEuxTa1-yNbyO4,研究该体系中紫外光和X射线激发下的发光性能,研究表明,在紫外光激发下,YTaO4:Nb,Eu是一种比较有效的红色发光材料,激发能可以通过NbO^3 4离子传递给Eu^3 ,随钽酸盐中NbO^3-4基团浓度的增中,化合物的结构从M'型YTaO4变成褐钇铌型YNbO4结构,它的发光性质也随之改变。     

Competitive interactions of adriamycin and ethidium bromide with DNA as studied by full rank parallel factor analysis of fluorescence three-way array data

The competitive interactions of adriamycin (AMC) and a fluorescence probe of ethidium bromide (EB) with DNA have been studied by full rank parallel factor analysis (FRA-PARAFAC) of fluorescence excitation-emission three-way data array. The excitation and emission spectra as well as the equilibrium concentrations of co-existing species in different reaction mixtures can be directly obtained by the FRA-PARAFAC treatment. The concordance of the resolved excitation and emission spectra of AMC, EB and EB-DNA with the standard spectra of these species confirmed the reliability of the equilibrium concentrations of these components in the reaction mixtures studied. The results obtained are valuable for providing a deeper insight into the competitive interaction mechanism of AMC and EB with DNA. The conclusion was directly given out that the interaction of AMC with DNA is the intercalating model. The FRA-PARAFAC method as exemplified by the present study provides an useful approach for studying the interaction of clinical drugs with DNA in the presence of disturbance of drug assistants.     

The spectrofluorqmetric determination of anthracene, fluorene and phenanthrene in mixtures

A procedure is presented for the spectrofluorometric determination of mixtures of anthracene, fluorene and phenanthrene. The determination depends on differences in fluorescence emission spectra and on selective excitation of anthracene fluorescence. Some of the fluorescence and absorption spectra involved overlap, but these difficulties can be overcome by empirical corrections. The average relative error in this method is less than 5 % over the concentration range 0 to 5 ppm.     

Shape and position of 4-aminophthalimide (4-AP) time-resolved emission spectra (TRES) versus sodium dodecyl sulfate sds concentration in micellar solutions: the partitioning of 4-AP in the micellar phase and in water surrounding the micelles

Time-resolved emission spectra (TRES) of 4-aminophthalimide (4-AP) dissolved in water solutions of sodium dodecyl sulfate (SDS) for three surfactant concentrations (0.05, 0.15, and 0.45 M) have been determined. The fraction of 4-AP dissolved in the water phase surrounding the micelles has been shown to increase with decreasing concentration of the surfactant. To obtain TRES of 4-AP present exclusively in micelles, a method of subtraction of the contribution of the emission originating from 4-AP present in the water phase surrounding the micelles from the total emission of the probe dissolved in SDS solution has been proposed. The consequences of failing to take into account the partitioning of 4-AP between the water and micellar phases are illustrated by some exemplary TRES results, taken before and after the subtraction of the emission originating from 4-AP present in the water phase. Together with the time of appearance and presence of isoemissive points in the time-resolved area-normalized emission spectra (TRANES), these results have shown a clear dependence of the rate and character of the 4-AP TRES changes on the SDS concentration. In connection with our earlier results and literature data, it has been concluded that the concentration of the water solubilized in micelles is the main factor determining the rate and character of these changes.     

Radiative relaxation of Sepia eumelanin is affected by aggregation.

The steady‐state and time‐resolved emission properties of aqueous solutions containing different aggregation state distributions of eumelanin are reported. Excitation spectra of the size‐selected samples reveal, for the first time, differences in absorption bands due to varying levels of aggregation. These size‐dependent absorption properties result in size‐dependent emission band shapes and quantum yields. For all size fractions, absorption and emission overlap significantly. The emission yield for small eumelanin aggregates is 5.7 times greater than that for large eumelanin aggregates. Time‐resolved population decays reveal that small eumelanin aggregates are responsible for long‐lived emission dynamics (lifetimes greater than 1 ns), while large eumelanin aggregates are the source of short emission decay (lifetimes less than 1 ns). Polarized emission decays for the large and small aggregates reveal that energy transfer occurs both within the same and between the separate fundamental building blocks of eumelanin. The observed energy transfer dynamics can be accounted for using Förster theory.